R03: Genetic Control of Post-Embryonic Developmental Progression in Zebrafish Project Summary/Abstract We still very little about the mechanisms of morphogenesis and differentiation during post- embryonic developmental stages in vertebrates. Nonetheless, understanding the factors regulating these later developmental periods is essential to understanding how adult traits form, and will lend insight into morphological defects and disorders that arise during human post- embryonic fetal and neonatal periods. This research utilizes the zebrafish, which undergoes extensive post-embryonic development involving changes in a variety of organ systems, changes similar or identical to processes that occur in prenatal humans. This proposal takes a two-pronged strategy towards understanding the genetic controls of the larval-to-adult transition in zebrafish. The first aim adopts a targeted approach, concentrating on two mutants that undergo complete somatic arrest during larval development, arresting after two and three weeks of development, respectively. These phenotypes suggest an impairment of genes absolutely required for post-embryonic developmental progression; mapping and cloning the mutations and characterizing the pathways will reveal some of the genes required for developmental processes occurring after embryogenesis. Since so little is known about the molecular changes involved in post-embryonic developmental progression, the second aim of this proposal represents an exploratory strategy that will extensively characterize transcriptional changes that occur during key developmental transitions. Transcriptomes will be produced for the skin, which undergoes well-characterized tissue-level transformations at post-embryonic stages, and the brain, which undergoes extensive neuronal maturation and neurogenesis during larval-to-adult transformation. Further, many of the global endocrine cascades that regulate growth and development originate in the brain. Transcript abundance will be quantified at different stages in each organ for both known and unknown mRNAs, including any splice variants. Since microRNAs are so crucial in regulating stage transitions in invertebrates, temporal expression of small, non-coding RNAs will also be fully characterized. These post-embryonic developmental transcriptomes will serve as resources to the larger research community and will allow detection of molecules and pathways that may mediate post-embryonic transitions. Dynamic expression profiles will further allow full characterization of the molecular phenotypes of the developmental arrest mutants. Overall, these efforts will generate resources for future studies of post- embryonic transitions and will identify novel factors that regulate and are required for normal progression through post-embryonic developmental stages.